Myotubularin related protein 2 (MTMR2) is a phospho-lipid phosphatase specific for the 3 positions of PI(3)P and Pl(3,5)P2. Loss of function mutations in MTMR2 cause the demyelinating peripheral neuropathy Charcot-Maire-Tooth disease type 4B (CMT-4B), a severely debilitating, hereditary condition characterized by focally folded myelin sheaths. Recessive mutations in a second myotubularin family member, MTMR13 (also called SET domain binding factor 2 [SBF2]) also cause CMT-4B. MTMR13 is a catalytically inactive member of the myotubularin inositol lipid phosphatase family. Recent data from our laboratory indicate that MTMR2 also binds to and is regulated by the closely related protein MTMR5 (also called SBF1). MTMR5 is 59% identical to MTMR13, has an identical domain composition, and is also an inactive phosphatase. However, MTMR5 is unlikely to be involved in CMT-4B. We propose to investigate how the lipid phosphatase activity of MTMR2 is regulated. Specifically, the effect of MTMR13 on MTMR2 function will be examined. Initial experiments will involve investigating whether the MTMR2 and MTMR13 proteins physically interact. The effect of MTMR13 association on the catalytic activity of MTMR2 will be analyzed both in vitro and in cell biological experiments. The effect of MTMR2-MTMR13 interactions on the cellular localization of both proteins will also be investigated. Finally, experiments are proposed to identify additional proteins that associate with MTMR13 and MTMR2.